1. Field of the Invention
The present invention relates to a regenerator (cold accumulating unit) filled with a cold accumulating material and a cold accumulating type refrigerator using the regenerator, and more particularly to a regenerator of which cold accumulating material is free from the risk of being pulverized into fine particles, and is excellent in workability and durability, and exhibits significant refrigerating performance at a low temperature region, and relates to a cold accumulation refrigerator using the regenerator (cold accumulating unit).
2. Description of the Related Art
Recently, superconductivity technology has been progressed remarkably and with an expanding application field thereof, development of a small, high performance refrigerator has become indispensable. For such a refrigerator, light weight, small size and high heat efficiency are demanded, and a small-sized refrigerator has been practically applied to various industrial fields.
For example in a super-conductive MRI apparatus, cryopump and the like, a refrigerator based on such refrigerating cycle as Gifford MacMahon type (GM refrigerator), Starling method, pulse-tube type refrigerator has been used. Further, a magnetic floating (levitating) train absolutely needs a high performance refrigerator for generating magnetic force by using a super-conductive magnet. Further, in recent years, a super-conductive power storage apparatus (SMES) or an in-magnetic-field single crystal pull-up apparatus (magnetic field applied Czochralski) has been provided with a high performance refrigerator as a main component thereof.
In the above described refrigerator, the operating medium such as compressed He gas or the like flows in a specified direction in a regenerator (cold accumulating unit) filled with cold accumulating materials so that the heat energy thereof is supplied to the cold accumulating material. Then, the operating medium expanded here flows in an opposite direction and receives heat energy from the cold accumulating material. As the recuperation effect is improved in this process, the heat efficiency in the operating medium cycle is improved so that a further lower temperature can be realized.
As a cold accumulating material for use in the above-described refrigerator, conventionally Cu, Pb and the like have been used. However, these cold accumulating materials have a very small specific heat in extremely low temperatures below 20K. Therefore, the aforementioned recuperation effect is not exerted sufficiently, so that even if the refrigerator is cyclically operated under an extremely low temperature, the cold accumulating material cannot accumulate sufficient heat energy, and it becomes impossible for the operating medium to receive the sufficient heat energy. As a result, there is posed a problem of that the refrigerator in which the regenerator (cold accumulating unit) filled with aforementioned cold accumulating material is assembled cannot realize the extremely low temperatures.
For the reason, recently to improve the recuperation effect of the regenerator at extremely low temperature and to realize temperatures nearer absolute zero, use of magnetic cold accumulating material made of intermetallic compound formed from a rare earth element and transition metal element such as Er3Ni, ErNi, ErNi2, HoCu2 having a local maximum value of volumetric specific heat and indicating a large volumetric specific heat in an extremely low temperature range of 20K or less has been considered. By applying the magnetic cold accumulating material t o the GM refrigerator, a refrigerating operation to produce an arrival lowest temperature of 4k is realized.
The magnetic cold accumulating material described above is normally worked and used in a form of spherical-shape having a diameter of about 0.1-0.5 mm for the purpose of effectively performing the heat exchange with He gas as cooling medium in the refrigerator. In particular, in a case where the magnetic cold accumulating material (particulate cold accumulating substance) is intermetallic compound containing rare earth element, the particulate cold accumulating substance is worked so as to have a spherical-shape in accordance with working methods such as centrifugal atomizing method.
However, in a Starling-type refrigerator and a pulse-tube type refrigerator or the like to be operated with a high speed, there has been posed a problem that a pressure loss at the regenerator packed with spherical magnetic cold accumulating particles is disadvantageously increased, so that a sufficient refrigerating capacity cannot be realized. Further, in the GM refrigerator or the like, there has been liable to cause the following disadvantages. Namely, vibration and impact force are applied to the magnetic body particles (magnetic cold accumulating particles) during the operation of the refrigerator and the magnetic particles were liable to be further finely pulverized, so that a flow resistance of the cooling medium gas is increased thereby to abruptly lower the heat exchange efficiency.
To cope with these problems, as samples of structure of cold accumulating material for lowering the pressure loss of the cooling medium gas, there has been proposed: a cold accumulating material composed of a punching plate formed by punching a magnetic material plate so as to form a number of through holes through which the cooling medium gas flows; a cold accumulating material composed of a rolled ribbon formed by winding a magnetic material ribbon; and a cold accumulating material composed of a screen formed by laminating a plurality of net-shaped magnetic materials.
However, since the cold accumulating materials described above exhibit a brittleness peculiar to the intermetallic compound, there had been raised a problem such that it was difficult to punch, bend or drill the materials, and it was extremely difficult to work the materials to have the above shapes, and the materials required an enormous large amount of working cost.
The present invention has been achieved to solve the above described problems and an object of the invention is to provide a regenerator (cold accumulating unit) filled with cold accumulating material which is free from the fear of being finely pulverized, and is excellent in workability and durability, and capable of exhibiting a significant refrigerating performance at an extremely low temperature range for a long period of time in a stable condition, and provide a cold accumulation refrigerator using the same.
In addition, another object of the present invention is to provide an MRI apparatus, a super-conducting magnet for magnetic floating train, a cryopump and an in-magnetic field single crystal pull-up apparatus capable of exerting an excellent performance for a long period of time by using the aforementioned cold accumulation refrigerator.
To achieve the above objects, the regenerator (cold accumulating unit) of the present invention comprises a regenerator body and cold accumulating material packed in the regenerator body in which cooling medium gas flows from one end portion of the regenerator body to the other end portion of the regenerator body so as to obtain a lower temperature, wherein at least part of the cold accumulating material is a plate-shaped cold accumulating material having a thickness of 0.03-2 mm.
Further, in the above structure, it is preferable that the cold accumulating material is composed of an alloy containing 10 at % or more of rare earth element, and that a length of the plate-shaped cold accumulating material in a flowing direction of the cooling medium gas is set to 1-100 mm. In addition, it is also preferable that a plurality of the plate-shaped cold accumulating material are arranged in a direction normal to the cooling medium gas flowing direction so as to form gaps therebetween, and a width of the gap is 0.01-1 mm.
Furthermore, in the above regenerator (cold accumulating unit), it is preferable to constitute the regenerator so that grooves are formed to an inner surface of the regenerator body, and a peripheral portion of the plate-shaped cold accumulating material is inserted in the groove. In addition, it is also preferable that projections are formed to an inner surface of the regenerator body, and a peripheral portion of the plate-shaped cold accumulating material is inserted into a portion between the projections. Further, it is also preferable that a plurality of the plate-shaped cold accumulating materials are fixed by a retainer, and the retainer is inserted in the regenerator body. Furthermore, it is also preferable that a plurality of the plate-shaped cold accumulating materials are arranged in a cooling medium gas flowing direction, and an angle constituted by a plane surface of the plate-shaped cold accumulating material and a plane surface of an adjacent plate-shaped cold accumulating material arranged in a cooling medium gas flowing direction is set to 0.5xc2x0 or more in a radial direction of the regenerator.
In addition, as a special construction of a regenerator (cold accumulating unit), the regenerator can be also constituted such that a plurality of the plate-shaped cold accumulating materials are arranged so as to partition a cross sectional area of a flowing passage of the cooling medium gas thereby to form a plurality of cells through which the cooling medium gas flows. In the above structure, it is preferable that the cold accumulating material forming the cell has an average thickness of 0.05-2 mm. Further, it is also preferable that a plurality of the cells have an average cross-sectional area of 1xc3x9710xe2x88x929 m2 to 2xc3x9710xe2x88x926 m2. Furthermore, it is also preferable that a plurality of the cells have an average length of 3 mm to 100 mm.
In addition, it is also preferable that a plurality of the plate-shaped cold accumulating materials and the cells are formed through an extrusion of a mixture comprising a binder and cold accumulating material powder. In this case, it is preferable that the cold accumulating material powder contains 10 at % or more of rare earth element.
The cold accumulation refrigerator of the present invention is characterized by comprising a regenerator (cold accumulating unit) filled with a cold accumulating material through which a cooling medium gas flows from a high temperature-upstream side of the regenerator, so that heat is exchanged between the cooling medium gas and the cold accumulating material thereby to obtain a lower temperature at a downstream side of the regenerator, wherein at least part of the regenerator (cold accumulating unit) is composed of the regenerator as described above.
Each of the MRI (Magnetic Resonance Imaging) apparatus, superconducting magnet for the magnetic floating train, cryopump and in-magnetic field single crystal pull-up apparatus (magnetic field applied Czochralski) according to the present invention is characterized by comprising the cold accumulation refrigerator as described above.
It is preferable that at least part of the cold accumulating material to be packed in the regenerator body of this invention is formed of a magnetic alloy containing 10 at % (atomic %) or more of rare earth element.
To put it concretely, for example, it is preferable that the alloy constituting the cold accumulating material consists of a simple substance of rare earth element or intermetallic compound expressed by a general formula:
RMzxe2x80x83xe2x80x83(1)
wherein R denotes at least one of rare earth element selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb, while M denotes at least one element selected from the group consisting of Ni, Co, Cu, Ag, Al, Ru, In, Ga, Ge, Si and Rh, and z in atomic ratio satisfies a relation: 0xe2x89xa6zxe2x89xa69.0.
As is clear from the general formula (1) of RMz (0xe2x89xa6zxe2x89xa69.0), the cold accumulating material to be packed in the regenerator (cold accumulating unit) of the present invention is preferably constituted by magnetic substances such as a single substance of rare earth element or intermetallic compound containing rare earth element. In this regard, other than the magnetic substances described above, the cold accumulating material constituted by metallic materials such as Pb, Pb alloy, Cu, Cu alloy, stainless steel or the like can be also used together with the aforementioned magnetic substances.
In the general formula described above, R component denotes at least one of rare earth element selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Ho, Dy, Er, Dy, Tm and Yb, while M component denotes at least one element selected from the group consisting of Ni, Co, Cu, Ag, Al, Ru, In, Ga, Ge, Si and Rh.
When a mixing ratio z of M component with respect to R component exceeds 9.0, a proportion of rare earth element as magnetic element is remarkably lowered thereby to reduce the specific heat of the cold accumulating material. In this regard, in case of z=0, i.e. the cold accumulating material is composed of single substance of rare earth element, it is difficult to control the temperature exhibiting a high specific heat, so that the cold accumulating material is preferably composed of intermetallic compound containing rare earth element.
A preferable range of z is 0.1xe2x89xa6zxe2x89xa65, and more preferably be 1xe2x89xa6zxe2x89xa63. Particularly preferable concrete compositions may include Er3Ni, Er3Co, ErNi, ErNi0.9Co0.1, HoCu2, Erln3, HoSb, Ho2Al. In the above compositions as like ErNi0.9Co0.1 which is prepared by substituting Co for a part of Ni of ErNi, when a part of R component is substituted for at least one element of the other R component, or when a part of M component is substituted for at least one element of the other M component, it becomes possible to shift a temperature position of the volumetric specific heat peak of the magnetic substance, and to control a width of the specific heat peak so as to realize a specific heat which is effective as the cold accumulating material.
The cold accumulating material used in the present invention may be constituted by a molded body composed of a number of magnetic particles mainly comprised of oxide having a specific heat peak at an extremely low temperature region of 20K or less. As examples of the oxides constituting the magnetic particle, for example, the compositions having the following general formulas of (2), (3), (4) and (5) can be preferably used.
That is, there can be used: a perovskite type oxide expressed by a general formula of
RMxe2x80x22O3xe2x80x83xe2x80x83(2)
wherein R denotes at least one of rare earth element selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb, while Mxe2x80x2 denotes at least one element selected from 3B family elements in the long-periodic table;
a spinel type oxide expressed by a general formula of
AB2O4xe2x80x83xe2x80x83(3)
xe2x80x83wherein A denotes at least one element selected from 2B family elements, while B denotes at least one element selected from transition metal elements containing at least of Cr;
an oxide expressed by a general formula of
CD2O6xe2x80x83xe2x80x83(4)
xe2x80x83wherein C denotes at least one element selected from Mn and Ni, while D denotes at least one element selected from Nb and Ta; and
an oxide expressed by a general formula of
Gd1xe2x88x92xRxA1xe2x88x92yByO3xe2x80x83xe2x80x83(5)
xe2x80x83wherein R denotes at least one of rare earth element selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Tb, Dy, Ho and Er, while A denotes at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Al and Si, at least two elements being selected as A component in a case of x=0 and y=0, while at least one element being selected as A component in a case of x 0 or y 0, B denotes at least one element selected from the group consisting of Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Au, and Bi, and x in atomic ratio satisfies a relation: 0xe2x89xa6xxe2x89xa60.4, while y in atomic ratio satisfies a relation: 0xe2x89xa6xxe2x89xa60.4.
Regarding the general formula (5) of Gd1xe2x88x92xRxA1xe2x88x92yByO3, in a case of x=0 and y=0, the general formula (5) can be expressed by a formula of GdAO3. In this oxide composition of GdAO3, however, when the A component is composed of a single element, there can be generally obtained a magnetic body having a specific heat at an extremely low temperature region, while the magnetic body rarely exhibits a high specific heat at the extremely low temperature range of 4-6 K. Therefore, in a case of x=0 and y=0, at least two elements are selected as A component. On the other hand, when a part of Gd is substituted for the other rare earth element, or when a part of A component is substituted for the other element, it becomes possible to control the specific heat characteristics of the magnetic body thereby to obtain a cold accumulating material having an excellent performance.
In the above general formula (5) of a general formula of Gd1xe2x88x92xRx A1xe2x88x92y ByO3, R component denotes at least one of rare earth element selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Tb, Dy, Ho and Er, and R is an effective component for broadening a sharpened specific heat peak and controlling the position of the peak temperature. The R component is added so as to substitute a part of Gd. When the addition ratio x indicating the substituting amount of R component exceeds 0.4, the specific heat of the magnetic body is disadvantageously lowered. Among the above R component, Tb, Dy, Ho and Er are preferable, and Tb and Dy are more preferable.
Further, A component denotes at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Al and Si, and has an effect of controlling the peak of specific heat. At least two elements are selected in a case of x=0 and y=0, while at least one element is selected in a case of x 0 or y 0, so that a part of Gd or A component in GdAO3 type magnetic body is invariably substituted for the other element. Among the above a component elements, of Ti, V, Cr, Mn, Fe, Co, Ni, Ga and Al are preferable, and Cr, Mn, Fe, Co, Ni, Ga and Al are more preferable.
Furthermore, B component is an element for improving the specific heat characteristic by the function of controlling a distance between atoms of (Gd1xe2x88x92xRx) when B component is substituted for a part of A component. The B component denotes at least one element selected from the group consisting of Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Au and Bi. As the B component element, Zr, Nb, Mo, Sn, Ta and W are preferable, and Ta and W are more preferable. When the addition ratio y indicating the addition amount of B component exceeds 0.4, it becomes impossible to maintain the perovskite structure, so that the specific heat characteristics of the cold accumulating material composed of the magnetic body is disadvantageously lowered.
Furthermore, there may be a case where the atomic ratio of oxygen in the above general formula: Gd1xe2x88x92xRx A1xe2x88x92y ByO3 is deviated from a stoichiometric ratio of 3 due to atomic defectives or the like. However, if the atomic ratio of oxygen is within a range of 2.5-3.5, the above deviation has not a great influence on the specific heat characteristic of the magnetic body.
A method of manufacturing the plate-shaped cold accumulating material to be packed in the regenerator (cold accumulating unit) of the present invention is not specifically limited. For example, a working method in which an alloy ingot of a cold accumulating material having the above composition is cut and sliced by means of cutting tool such as a blade saw or the like, or a powder-sintering method or the like can be used.
Further, in a case of a cold accumulating material formed with a plurality of cells through which a cooling medium gas flows, such the cold accumulating material can be formed through an extrusion of a mixture of cold accumulating material powder and a binder, as described later.